The present invention relates generally to automatic gain control apparatus, and, more particularly, to an automatic gain control system for a receiver capable of controlling signal levels of either a constant envelope information signal, or a non-constant envelope information signal when received thereat.
A radio communication system is comprised, at minimum, of a transmitter and a receiver. The transmitter and the receiver are interconnected by a radio-frequency channel to permit transmission of an information signal therebetween.
Typically, the information signal is impressed upon a radio-frequency electromagnetic wave by a process referred to as modulation to permit transmission of the information signal between the transmitter and the receiver. The radio-frequency electromagnetic wave is referred to as a carrier wave which is of a particular frequency, and the carrier wave, once modulated by the information signal, is referred to as a modulated, information signal. The modulated information signal may be transmitted through free space to transmit thereby the information between the transmitter and the receiver.
Various modulation techniques have been developed to modulate the information signal upon the electromagnetic wave. Amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), and composite modulation (CM) are four of such modulation techniques.
In general, an amplitude modulated signal is formed by impressing (i.e., modulating) an information signal upon a carrier wave such that the information signal modifies the amplitude of the carrier wave corresponding to the value of the information signal. Amplitude modulation does not cause the frequency of the carrier wave to vary, and the information portion of the modulated information signal is contained in the shape, i.e., amplitude, of the signal. The shape of the modulated information signal is referred to as the envelope of the signal, and the changes in the amplitude of the information signal change the shape of the envelope formed thereby.
A frequency modulated signal formed is formed by impressing (i.e., modulating) an information signal upon a carrier wave such that the information signal modifies the frequency of the carrier wave corresponding to the value of the information signal. Frequency modulation does not cause the amplitude of the carrier wave to vary, and the information content of the modulated information signal is contained in the variation of the frequency of the signal. Because the amplitude of a frequency modulated signal does not vary, a frequency modulated signal is referred to as a constant envelope signal.
A phase modulated signal is formed by impressing (i.e., modulating) an information signal upon a carrier wave such that the information signal modifies the phase of the carrier wave corresponding to the value of the information signal. Phase modulation does not cause the amplitude of the carrier wave to vary. The information content of the modulated information signal is contained in the variation of the phase of the signal. Because the amplitude of a phase modulated information signal, similar to that of a frequency modulated signal, does not vary, a phase modulated signal is referred to as a constant envelope signal.
A composite modulated signal is formed by impressing (i.e., modulating) an information signal upon a carrier wave such that the information signal modifies both the amplitude and the phase of the carrier wave. Conventionally, in order to form the composite modulated signal, the carrier wave is first separated into sine wave and cosine wave component portions. Separate portions, referred to as the in-phase (or I) and the quadrature (or Q) components, of the information signal are impressed upon the cosine wave and sine wave component portions of the carrier wave. (More particularly, the in-phase component of the information signal is impressed upon the cosine wave component of the carrier wave, and the quadrature component of the information signal is impressed upon the sine wave component of the carrier wave.) The sine wave and cosine wave components are then recombined, and the resultant signal, the composite modulated signal, varies in both amplitude, and, additionally, phase. Composite modulation is advantageous in that a composite modulated signal permits a greater amount of information to be transmitted within a frequency bandwidth than a signal generated by any of the previously mentioned modulation techniques.
A receiver which receives a modulated information signal, such as a one formed by one of the above-described modulation techniques, includes circuitry to detect, or otherwise to recreate, the information signal modulated upon the carrier wave. This process is referred to as demodulation. As many different modulated information signals may be simultaneously transmitted by a plurality of transmitters at a plurality of different frequencies, a receiver contains tuning circuitry to demodulate only those signals received by the receiver which are of certain desired frequencies. The broad range of frequencies at which modulated information signals may be transmitted is referred to as the electromagnetic frequency spectrum. Regulation of radio-frequency communications in certain frequency bands of the electromagnetic frequency spectrum minimizes interference between simultaneously transmitted signals.
For example, portions of a 100 MHz band of the electromagnetic frequency spectrum (extending between 800 MHz and 900 MHz) are allocated for radiotelephone communication, such as, for example, communication effectuated by radiotelephones utilized in a cellular, communication system. Existing radiotelephones contain circuitry both to generate and to receive radio-frequency modulated information signals.
A cellular, communications system is created by positioning numerous base stations at spaced-apart locations throughout a geographical area. Each of the base stations is constructed to receive and to transmit modulated information signals simultaneously to and from radiotelephones to permit two-way communication therebetween.
The base stations are positioned at locations such that a radiotelephone at any location throughout the geographical area is within the reception range of at least one of the base station receivers. The geographical area is divided into portions, and one base station is positioned in each portion. Each portion of the geographical area defined thereby is referred to as a "cell".
Although numerous modulated information signals may be simultaneously transmitted at different transmission frequencies, each modulated information signal, when transmitted, occupies a finite portion of the frequency band. Overlapping of simultaneously transmitted modulated, information signals in the same geographic area is impermissible as interference between overlapping signals at the same frequency could prevent detection of either of the transmitted modulated information signals by a receiver.
To prevent such overlapping, the frequency band allocated for radiotelephone communication is divided into channels, each of which is of a 30 KHz bandwidth. A first portion, extending between 824 MHz and 849 MHz of the frequency band, is allocated for the transmission of modulated information signals from a radiotelephone to a base station. A second portion, extending between 869 MHz and 894 MHz of the frequency band is allocated for the transmission of modulation information signals from a base station to a radiotelephone.
Increased usage of cellular, communication systems has resulted, in many instances, however, in the full utilization of every available transmission channel of the frequency band allocated for cellular, radiotelephone communication. Other frequency bands of the electromagnetic frequency spectrum are oftentimes similarly fully utilized.
Various attempts have been made to utilize more efficiently the frequency band allocated for radiotelephone communications to increase thereby the information transmission capacity of a cellular, radiotelephone communication system. Attempts have been similarly made to use more efficiently other frequency bands of the electromagnetic frequency spectrum.
Conventionally, the modulation technique utilized by radiotelephone communication systems to form the modulated information signal thereby is frequency modulation. As mentioned previously, a frequency modulated signal impresses an information signal upon a carrier wave to modify the frequency of the carrier wave according to the value of the information signal. However, conventional frequency modulation techniques form a continuous-wave, FM modulated signal, and only one such continuous-wave signal may be transmitted upon a transmission channel at a time.
Techniques have been developed, which permit transmission of more than one signal at the same frequency. One such technique involves the sequential time-sharing of a single channel by several radiotelephones. This technique is referred to as time-domain multiple access (or TDMA).
In order to use TDMA, an information signal (such as a voice signal) which is to be transmitted is first encoded according to an encoding scheme. Once encoded, the information signal, in encoded form, is modulated upon a carrier wave and is transmitted in intermittent bursts. Other information signals may similarly be encoded, modulated, and transmitted in intermittent bursts at the same frequency. Thus, a greater number information signals may be transmitted within a particular frequency bandwidth. When the information signals are generated by users of radiotelephones forming a portion of a cellular communications system, a greater number of radiotelephones may be operated within a particular frequency bandwidth when such a TDMA technique is utilized.
A receiver constructed to receive a TDMA signal, such as a TDMA composite modulated signal, reconstructs the original information signal by decoding the TDMA signal transmitted to the receiver in intermittent bursts.
A system which utilizes composite modulated signals which are transmitted utilizing the TDMA technique has been chosen for a system to augment existing U.S. domestic cellular telephone systems. Compatability between existing domestic cellular telephone systems and the proposed system is necessary to permit equipment constructed for use on the proposed system also to be used on the existing systems. Thus, radiotelephones are being constructed which are capable of receiving both FM, continuous-wave signals, and TDMA composite-modulated signals. Such radiotelephones may be suitably operated in a conventional cellular communication system which utilizes FM continuous-wave signals, and in a cellular, communication system which utilizes TDMA composite-modulated signals.
A receiver constructed to receive TDMA composite-modulated signals may also require circuitry to perform equalization in the receiver. Equalizer circuitry is required to correct for delay problems associated with reflections of signals transmitted to the receiver which arrive at the receiver at different times. Because the signal received by a receiver is actually a vector sum of all signals transmitted at a particular frequency, the signal received by a receiver may actually be comprised of the same signal at different times as the signal may be reflected off objects prior to reception thereof by the receiver. The signal actually received by the receiver is, therefore, the sum of all signals which are transmitted to the receiver along many different paths. The path lengths may vary, and hence the signal actually received by the receiver may vary, responsive to repositioning of the receiver. Equalizer circuitry is oftentimes formed by a processor having an appropriate software algorithm embodied therein. In order to permit optimal operation of the equalizer circuitry, the receiver should be constructed to be linear (i.e., the demodulated signals should represent accurately the original I and Q portions modulated onto the carrier).
The linearity of a receiver defines the efficiency of the recreation of a received signal. An ideal receiver reproduces only the signal transmitted thereto. Actual, nonideal receivers, through a process of amplification and mixing occurring during frequency conversion of a received signal, produce intermodulation distortion. Associated with intermodulation distortion are undesired spurious signals generated during frequency conversion of a signal received by a nonideal receiver. Such undesired, spurious signals are referred to hereinbelow as intermodulation spurs. A highly nonlinear receiver generates a large amount of intermodulation distortion.
Typically, receivers, including those utilized in a conventional, cellular radiotelephone communication systems, minimize the deleterious effects caused by the generation of intermodulation spurs by including, as a portion of the receiver circuitry, filter circuits to filter the undesired signals and reduce the level of intermodulation spurs generated during frequency conversion of a received signal. Such filters may be comprised of either active or passive filter stages. An active filter stage may be advantageously embodied in an integrated circuit, but an active filter is generally linear over only a limited dynamic range of received signals. Additionally, an active filter exhibits proper filter characteristics over only the limited dynamic range.
As noted hereinabove, because a modulated information signal transmitted at a particular frequency may be reflected off objects prior to reception thereof by a receiver, the signal received is actually the sum of many signals received from many different paths. Hence, the signal level (i.e. amplitude) of the received signal is actually the vector sum of many signals received from many paths. The number of, and intensity of, signals actually received by a receiver may vary over time as a result of repositioning of the receiver, or of the objects from which a transmitted signal is reflected. As a result, the signal level of a received FM signal varies over time. This variance is referred to as "fading" of the signal. The rate at which the resultant signal strength at the receiver varies is predominantly determined by how rapidly the receiver is moving through its environment, and the frequency of the channel being used. For instance, in the cellular frequency band, and when a cellular radiotelephone is positioned in a vehicle travelling at sixty miles per hour, the signal strength of the received signal can vary by approximately twenty decibels during a five millisecond period.
Conventional FM receivers utilize voltage limiters prior to signal demodulation which clip the received signal. The resultant signal is of a constant envelope, and the deleterious effects of fading are thereby minimized. Since the information in an FM signal is not carried in the envelope, clipping the received signal to form a signal of a constant envelope permits optimal recovery of the frequency modulation, and, hence, the information content, of the received signal. Gain control of an FM-only, continuous-wave receiver is not necessary for demodulation, though such gain control may be utilized to adjust the received signal level to permit optimal operation of amplification and filtering circuitry within the receiver.
Receivers constructed to receive a TDMA composite-modulated information signal, however, require gain control circuitry to correct for the effects of changes in signal levels induced by fading, and to allow recovery of the information components encoded in the envelope of the signal.
Because radiotelephones being constructed to permit demodulation of TDMA composite-modulated signals are also to permit demodulation of conventional, continuous-wave signals, the radiotelephone must be constructed to contain gain control circuitry to correct for the effects of changes in signal levels of TDMA composite-modulated signals. Such gain control circuitry may also serve to ensure optimal performance of the receiver during reception of continuous-wave signals. The form and performance of gain control circuitry may vary, though, depending on the type of modulation being received.
When a composite modulated signal is received, gain control circuitry should be of a design to permit rapid and continuous tracking of variations in received signal levels due to fading. In addition, a radiotelephone which generates a TDMA composite modulated signal to transmit an information signal in a cellular communication system also measures intermittently the signal strengths of transmitters located in one or more cells. This process of testing signal strengths is referred to as mobile-assisted hand-off (or MAHO). The MAHO test also requires gain control circuitry which permits rapid and continuous tracking of a signal.
Digital signal processors may be utilized to form such rapid-tracking, gain control circuitry. However, digital signal processors require significant amounts of power for operation. Cellular, radiotelephone equipment may be battery powered; for such equipment, continuous use of digital signal processing circuitry to perform gain control may create an undesirable power load on the battery when the receiver receives a continuous-wave FM signal.
When receiving an FM-modulated signal, the gain control circuitry need not be of a design to track fading (i.e., the gain control circuitry need not permit rapid and continuous tracking). A normal FM limiter demodulator is insensitive to the variations induced by fading, and the aforementioned MAHO operation is not performed during continuous-wave reception. For continuous-wave reception, slow-responding gain control circuitry constructed with analog circuit elements which require only low power for operation thereof is possible.
A radiotelephone operable to receive both conventional continuous-wave signals, and TDMA composite-modulated signals having gain control circuitry for controlling signal levels of either type of transmitted signal, and, additionally, having minimal power consumption requirements would be advantageous.
What is needed, therefore, is a gain control scheme which requires minimal power consumption, but which also may be alternately operated to control signal levels of either conventional, continuous-wave modulated information signals, or TDMA composite-modulated information signals transmitted to the radiotelephone.